Energy and Power in Electric Circuits – Learn

Potential difference is defined as the work done to move a charge against an electric field between two points. The equation relating potential difference and work is:

V=\cfrac { W }{ q } or W=Vq


V = voltage (in V)

W = work (in J)

q = charge (in C)

The rate at which energy is used or converted into other forms of energy over a period of time is known as its electrical power. Power (P) is the rate at which energy is transformed in a circuit component, and is measured in watts (W). Power is calculated using:

P=IV=\cfrac { E }{ t }


P = power (in W)

These equations are often rearranged to give the relationship: 


Example 1:

Calculate the work carried by 4C of charge with a potential difference of 12V:


Using: W=Vq

W=12\times 4

W= 48J 

Example 2:

A speaker operating on a voltage of 18V draws a current of 3A. Calculate the power used by this speaker:


Using P=IV

P=18\times 3


Example 3:

A potential difference of 6V is used to produce a current of 150mA in a set of headphones for 10 minutes. How much energy is used in this time?


Using: E=VIt

E=6\times 0.15\times 600